1. Technical Field
The present invention relates to a method of manufacturing magnetic recording medium having superior recording characteristic for short wavelength recording.
2. Prior Art
Hitherto for magnetic recording medium, coating type medium wherein magnetic powder coating are applied on a non-magnetic substrate sheet or film has been widely used. Recently the magnetic recording and playing apparatus become miniaturized and are highly integrated as general trend, and the above-mentioned coating type recording media has technical limit in fulfilling needs for such high density recording. In order to overcome such limit, recording media of ferro-magnetic metal thin-film has been increasingly considered. The present invention relates to an improvement in such ferromagnetic thin film recording media.
Among the thin film materials, those having Co as the host material is most superior since it has large crystal aniisotropy energy based on hexagonal close packed structure. Especially a Co-Cr film containing 10-30 weight percent Cr can be a superior vertical magnetization film (the film is hereinafter referred to Co-Cr vertical magnetization film). A Co-Cr film containing 10-30 weight percent Cr is superior also in corrosion-resistivity. That is, a magnetic recording medium having Co-Cr film containing 10-30 weight percent Cr has actual utility, and also superior in short wavelength recording characteristic, hence can be said as ideal thin film medium. Various methods exist for coating the ferromagnetic metal thin-film on the recording medium, including plating method, sputtering method, vacuum deposition method, or the like. The vacuum deposition method is most superior method for mass production. In order to form a thin film type recording medium by means of the vapour deposition method while maintaining good and stable manufacturing productivity, usually the vapour deposition is made on a substrate 1 consisting of a high polymer material being driven along a cylindrical can or cylindrical body 2 from a vaporizing source 5 in an evacuated chamber 100. The cylindrical can is driven to rotate around a shaft S by a known means. The high polymer substrate 1 is fed out from one roll 3 and wound up by another roll 4, as shown in FIG. 1.
However when the thin film is formed in the above-mentioned way, in general the recording medium is likely to curl as shown in FIG. 2(a) and in FIG. 2(b). That is, the recording medium curls concavely with respect to the ferromagnetic metal thin film 6 or convexly, depending on the natures and conditions of material and manufacturing processes. Accordingly the recording medium is likely to have insufficient contact with a magnetic head used for writing or reading information onto or from the medium, and hence poor characteristics in running or winding up. The state of curling shown in FIG. 2(a) is called normal curl and the state of curling shown in FIG. 2(b) is called inverse curl. Providing the width of the recording medium when it is flatly extended is l.sub.0, and is curled to become the width of l as shown in FIG. 2, then a ratio of curling R defined by EQU (l.sub.0 -l)/l.sub.0 =R
should be 4% or lower in order to be usable as magnetic recording medium. In order to fulfil the above-mentioned condition, it is known to form a counter curl coating 7 as shown in FIG. 3 on the opposite face to the ferromagnetic thin metal coating 6, for instance using non-magnetic coating, or alternatively when the curling is of normal curling, a heat treatment may be carried out after forming the thin ferromagnetic film so as to shrink the substrate; but either method requires additional process leading to increased production costs.